Nitration of aromatic compounds such as benzene and toluene to produce nitroaromatic compositions such as mononitrobenzene, dinitrobenzene, mononitrotoluene, dinitrotoluene, and trinitrotoluene has been carried out for a substantial period of time. Two techniques are primarily used on a commercial basis to produce mono and dinitrobenzene and nitrotoluenes. One nitration process involves the utilization of concentrated nitric acid, e.g., 95-98% nitric acid and the other involves the use of a mixed acid which comprises a mixture of sulfuric acid and nitric acid with the nitric acid concentration being about 63% to 67% and the sulfuric acid concentration being about 93% to 99% by weight. Both of the nitration processes generate a spent acid containing unreacted nitric acid, byproduct water and other impurities which must be removed from the product.
Various problems are associated with the recovery and concentration of nitric acid from spent acid by either nitration process, but recovery of nitric acid from spent acid generated in the mixed acid process and subsequent concentration of nitric acid and sulfuric acid for the nitration process, presents considerable problems. Several processes have been devised to recover nitric acid from this spent acid medium. Examples include:
U.S. Pat. No. 4,496,782 which discloses a process for denitrifying aqueous spent acid from the mononitration of aromatic compounds, e.g.. toluene. As noted in that patent, the recovery of the nitric acid from the spent mixed acid should be done with a minimum of processing to avoid risk of discoloration (charring) of the sulfuric acid, as the charred or black spent acid is unacceptable in the industry. One of the techniques for recovering the nitric acid from the spent acid, has been to carry out the reaction with an excess of aromatic nitratable compound. Reference was made to U.S. Pat. No. 4,021,498 and 2,849,497 which shows contacting a crude nitrobenzene product containing nitric acid with fresh benzene.
U.S. Pat. No. 4,650,912 discloses a process for denitrifying nitric acid and nitrous acid-containing spent acid obtained from the nitration of aromatic hydrocarbons by adding aromatic hydrocarbon to the spent acid phase and photometrically monitoring the denitrification reaction for the appearance of a dark red to black color. At that point the aromatic hydrocarbon-nitric acid molar ratio was adjusted to eliminate color by reducing aromatic hydrocarbon feed rate or adding nitric acid to the medium.
German publication P23 38 479.4 discloses a process for the workup of nitric acid containing nitration products obtained by the nitration of aromatic compounds by using the concentrated nitric acid process. Because of the problems associated with the recovery of nitric acid from a solution of an organic nitrocompounds and concentrated nitric acid, sulfuric acid was added to the nitric acid-nitro aromatic solution and then the nitric acid recovered by distillation.